Encoders provide a measurement of the position of a component in a system relative to some predetermined reference point. Encoders are typically used to provide a closed-loop feedback system to a motor or other actuator. For example, a shaft encoder outputs a digital signal that indicates the position of the rotating shaft relative to some known reference position that is not moving. A linear encoder measures the distance between the present position of a moveable carriage and a reference position that is fixed with respect to the moveable carriage as the moveable carriage moves along a predetermined path.
An absolute shaft encoder typically utilizes a plurality of tracks on a disk that is connected to the shaft. Each track consists of a series of dark and light stripes that are viewed by a detector that outputs a value of digital 1 or 0, depending on whether the area viewed by the detector is light or dark. An N-bit binary encoder typically utilizes N such tracks, one per bit. An incremental encoder typically utilizes a single track that is viewed by a detector that determines the direction and the number of stripes that pass by the detector. The position is determined by incrementing and decrementing a counter as each stripe passes the detector.
In both types of encoder, the ultimate resolution is determined by the stripe pattern and size of the detectors used to view the stripe pattern. To provide increased resolution, the density of the stripes must be increased. For example, in a shaft encoder, the number of stripes per degree of rotation must be increased. Similarly, in a linear encoder, the number of stripes between the limits of the linear motion must be increased. There is a practical limit, however, to the density of stripes that is set by optical and cost constraints.
In principle, some form of analog detection array based on analog interpolation can be used to define additional states. However, such systems have higher costs due to the additional analog circuitry. In addition, the analog encoding schemes are more susceptible to noise.